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The present invention relates to a vertical and short take-off and landing aircraft (V/STOL), and more particularly, the present invention relates to a V/STOL aircraft having ducted fans located in the aircraft""s wings that cooperate with the duct wall to provide improved vertical thrust to lift and hover aircraft.
As set forth in U.S. Pat. No. 4,828,203 to Robert T. Clifton and Woodrow L. Cook, current aircraft are generically divided into two major classes, fixed wing and rotary airfoil. The former are typically referred to as xe2x80x9cairplanesxe2x80x9d and derive their lift from the forward motion of the aircraft, causing air to pass over an airfoil. Rotary winged aircraft, commonly referred to as xe2x80x9chelicopters,xe2x80x9d have a prime mover attached to an airfoil which rotates.
The most successful V/STOL to date has been the helicopter. In addition to the prime mover, stub wings are sometimes added to produce lift at forward speeds. In hovering, however, the down wash of the rotor produces large loads on the wings, and compromises hovering performance. Another successful example of a V/STOL aircraft is the British Harrier military aircraft, which can rise vertically, and subsequently achieve supersonic speed in level flight. This is accomplished by running the aircraft""s jet engines in a horizontal position and deflecting the jet blast downwardly to effect vertical thrust for take-off.
Several V/STOL alternatives have been constructed and tested. In one alternative, the entire airplane is tilted. One example of a tilted aircraft is the Convair XFY-1 xe2x80x9cPogoxe2x80x9d which operates by tilting the power plant and wings in combination with one another. In another example, the Bell X-22, four tilting ducted propellers are utilized with a conventional stub wing mounted aft of the center of gravity. Another example is the Canadair CL-84 which features two prop-rotor engines mounted to a set of tilting wings. The current Bell-Boeing V-22 Osprey utilizes large powerplants pivoting on the wing ends of the xe2x80x9ctilt-rotorxe2x80x9d configuration. One problem with tilt-rotors involves stability control difficulties. Particularly, turbulent rotational flow on the prop-rotor blades may occur in descent and cause a vortex-ring state. The vortex-ring state causes unsteady shifting of the flow along the blade span, and may lead to roughness and loss of aircraft control. Also, the prop-rotors have a large diameter and may strike the landing surface when the engines are tilted fully forward. Current examples of V/STOL aircraft tested include the Boeing X-32B and Lockhead Martin X-35B. The Boeing X-32B utilizes turbojet engines with mid-fuselage lift nozzles, and the Lockhead Martin X-35B utilizes a separate lift fan driven by a drive shaft and clutch mounted on the prime mover. The U.S. Department of Defense selected the Lockhead Martin X-35B to serve as the military joint strike fighter.
Again, as set forth in U.S. Pat. No. 4,828,203 to Robert T. Clifton and Woodrow L. Cook, a number of common problems are associated with the currently known direct-lift aircraft. One problem is the detrimental effect of the high energy slip stream striking the ground. Loose material thrown about the aircraft constitutes a potential hazard to both the aircraft and to personnel in the vicinity of the aircraft. Another problem is the inadequate pitch control while in the hovering mode because of insufficient airflow over conventional control surfaces. A problem of the fan-in-wing aircraft is that the relative thin wings limit the depth of the fan duct and the vertical thrust produced by the fans.
The aircraft disclosed in the aforementioned U.S. Pat. No. 4,828,203 solves some of these problems. However, the aircraft disclosed in the patent generates a limited amount of thrust since air tends to leak (or flow) between the tips of the fan blades and the fan duct walls. Additionally, a lack of duct depth limits the amount of thrust that can be generated. In order to have sufficient depth to create the desired thrust efficiently, a relatively thick wing is needed. Moreover, the aircraft lacks the pitch control desired when the prime mover of the aircraft becomes inoperative and insufficient airflow moves across the canards and/or control surfaces mounted behind propeller. Also, during descent, the flow of air through the center of the fan duct may be too great, and induce a vortex state that causes a downward flow and pushes the aircraft rapidly and uncontrollably toward the ground. These and other problems are solved by the invention described below.
It is an object of the invention to provide a safer, more reliable and stable V/STOL aircraft.
It is another object of the invention to provide an aircraft which can land and take off in either conventional lift wing mode or lift fan mode.
Yet another object of the invention is to provide an aircraft allowing greater and more efficient airflow through the lift fan ducts, especially at vertical take off or landing.
Another object of the invention is to provide a lift improvement device which generates more effective airflow at the ducted fan blade tips.
Still another object is to provide an aircraft that resists or eliminates the undesirable vortex ring state effect.
Another object of the invention is to provide improved fan lifting performance by increasing the depth of the ducts with controllable duct extenders and other features.
Another object of the invention is to provide an aircraft having pitch control devices powered independently of the prime mover power, and allowing pitch control with separate pitch control fans without assistance from prime mover.
In accordance with these and other objects evident from the following description of a preferred embodiment of the invention, an aircraft is provided having a fuselage and a pair of main wings. Each main wing includes a lift fan segment, a generally circular duct defined within the lift fan segment and a fan mounted within the duct. A tip extender is coupled with the tip of at least one of the fan blades and contacts the duct sidewall so that leakage of air between the tip of the fan and duct sidewall is reduced and thrust efficiency increased.
In accordance with another aspect of the present invention, a fan-in-wing aircraft is provided that includes a duct extender located about the duct in the wing. The elongated duct extender is coupled with the lift fan segment of the wing and is capable of extension relative to the lift fan segment. When extended, the duct depth is increased causing air drawn through the duct and duct extenders to provide improved thrust of the lift fan segment.
In accordance with another aspect of the invention, a fan-in-wing aircraft is provided having a number of inlet control vanes pivotally coupled with the lift fan segment about the inlet of the duct, and a number of outlet control vanes pivotally coupled with the lift fan segment about the outlet of the duct. The outlet control vanes located near the center of the duct are operable independently of the remainder of the outlet control vanes to limit airflow through the center of the duct and prevent the inducement of a vortex ring state.
In yet another aspect of the invention, a fan-in-wing aircraft is provided having a pitch control assembly located forward of the center of gravity of the aircraft. The assembly includes a pair of canard wings disposed generally symmetrically about the fuselage and a pair of pitch control fans located proximate the canard wings. The flow of air over the canard wings or through one of a pair of outlets in each of the pitch control fans assists in controlling the pitch of the aircraft.